This invention relates to a hydraulic circuit for linearly driving a machine-tool slider in both directions.
Such a slider is for example a slider holding a movable roller in a pipe bending machine. However, such a slider can also belong to a press, to a bending machine, to a fixed radius pipe bender or another machine, in which such a slider must be moved to a certain position quickly and accurately. For simplicity and clarity sake a pyramidal, symmetrical pipe bending machine is referred to below as a machine tool.
The patent application PCT/IT 01/00381 of the same applicant provides a hydraulic circuit for linearly driving a movable roller-holder slider of a pipe bending machine, comprising an hydraulic cylinder whose piston rod is connected to such a slider that travels in its primary or work motion to a predetermined position for each pass of one or more passes of working operation of a workpiece to be bent, and in its return motion to a rest position, the hydraulic cylinder having a high pressure chamber and a low pressure chamber. Both chambers are communicating with respective ducts of pressurized fluid fed from a reservoir by a pump, ducts on which a three-position four-way valve and a check valve operate. The hydraulic circuit further comprises, between the three-position four-way valve and the check valve, a throttling valve, that is operated by an electromagnet to generate an increased pressure in the low pressure chamber in order to slow down the slider holding the upper roller in its primary motion when a programmable interval is approached from the predetermined position for each working pass.
The hydraulic circuit above mentioned allows the pressures between two chambers to be balanced, until to stop the slider exactly in the desired position in a unidirectional work travel, while in the other direction, or return travel of the slider, the stop accuracy of the same is coarse.
Thus, a problem of the stop accuracy of that slider when the return travel is also a work travel arises. This occurs for example, when an elongated workpiece must be bent in one or more passes with connections between contiguous curves having different radiuses. In these events the slider is necessarily moved to work positions in both directions.